1. Field of the Invention
The present invention relates to a two-stroke-cycle uniflow spark-ignition engine which is designed to generate power by supplying into a cylinder a scavenging fluid which has been precompressed in a crank chamber and which consists of an air-fuel mixture or air, while injecting fuel into the cylinder, if the scavenging fluid consists of the air, and by compressing the air-fuel mixture by means of a piston and ingiting the compressed air-fuel mixture by means of a spark, an exhaust gas being expelled through an exhaust valve provided in a cylinder head.
2. Description of the Related Art:
Conventional two-stroke-cycle spark-ingition engines are based on the so-called three-port type engine developed by Day, J., a British engineer, in 1891. In this two-stroke-cycle spark-ignition engine, the conversion of the energy provided by a fuel into useful power takes place as follows: An air-fuel mixture which is supplied from an inlet port of a cylinder is precompressed in a crank chamber, and the precompressed air-fuel mixture is introduced into the cylinder through a scavenging passage from a scavenging port which is opened to a cylinder surface against which a piston slides. The air-fuel mixture so supplied is further compressed by the piston in the cylinder. Upon ignition of this fuel-air mixture, it undergoes combustion and thereby generates power. Exhaust gas is expelled through an exhaust port provided in the surface of the cylinder against which the piston slides.
Suitable scavenging methods for such a conventional two-stroke-cycle spark-ignition engine include crossflow scavenging in which a scavenging port and an exhaust port are disposed in the cylinder in opposed relation with each other, and in which a piston head is provided with a protrusion to prevent the scavenging fluid from directly flowing into the exhaust port; and loop scavenging in which a plurality of scavenging ports are disposed symmetrically with respect to the exhaust port. The engine performance has been improved in both of these scavenging systems so that less fuel is required to operate the engine for a given load. However, it seems that the possibility for additional research has reached its limit.
As regards combustion, with any of the above-described scavenging systems, the rate of dilution of scavenging air (an air-fuel mixture) with residual combustion gas is far higher than that obtained in a normal four-stroke-cycle spark-ignition engine. This deteriorates the ignitability of the air-fuel mixture charged in the cylinder and makes it impossible for a two-stroke-cycle spark-ignition engine to be operated with an air-fuel mixture as thin as that used in four-stroke-cycle spark-ignition engines. Thus, misfire readily occurs unless a particularly powerful spark is used for ignition. Finding a solution to this problem is difficult because it is associated with the scavenging method employed.
Furthermore, the conventional two-stroke-cycle spark-ignition engine has essential disadvantages that it consumes a relatively large amount of lubricant, that the exhaust gas contains larger amounts of hydrocarbons and carbon monoxide due to the problems involving scavenging and combustion, and that the exhaust gas is smelly and accompanied by smother due to the fact that lubricant is readily mixed with the air-fuel mixture in the cylinder.
Although the conventional two-stroke-cycle spark-ignition engine generally has a slightly better engine performance, is simple and small in size, and can be manufactured at a lower cost than a four-stroke-cycle spark-ignition engine of the same displacement, it suffers from the problems that it consumes larger amounts of fuel and lubricant, that the exhaust gas is a pollutant, and that it generates vibrations and noise due to its lack of stability and smoothness of operation.
These factors limit the use of the existing two-stroke-cycle spark-ignition engine to special applications, including small portable industrial machines, small motor bicycles and motor boats, and excluding applications that require more power, such as automobiles, and those that require a low level of noise such as vehicles for use on public roads.